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<div id="dev-warning" class="admonition note">
  <p class="first admonition-title">Notice</p>
  <p class="last">This document is for a development version of Ceph.</p>
</div>
  <div id="docubetter" align="right" style="padding: 5px; font-weight: bold;">
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  <section id="d3n-rgw-data-cache">
<h1><a class="toc-backref" href="#id1" role="doc-backlink">D3N RGW Data Cache</a><a class="headerlink" href="#d3n-rgw-data-cache" title="Permalink to this heading"></a></h1>
<nav class="contents" id="contents">
<p class="topic-title">Contents</p>
<ul class="simple">
<li><p><a class="reference internal" href="#d3n-rgw-data-cache" id="id1">D3N RGW Data Cache</a></p>
<ul>
<li><p><a class="reference internal" href="#architecture" id="id2">Architecture</a></p></li>
<li><p><a class="reference internal" href="#implementation" id="id3">Implementation</a></p>
<ul>
<li><p><a class="reference internal" href="#requirements" id="id4">Requirements</a></p></li>
<li><p><a class="reference internal" href="#limitations" id="id5">Limitations</a></p></li>
</ul>
</li>
<li><p><a class="reference internal" href="#d3n-environment-setup" id="id6">D3N Environment Setup</a></p>
<ul>
<li><p><a class="reference internal" href="#running" id="id7">Running</a></p></li>
<li><p><a class="reference internal" href="#logs" id="id8">Logs</a></p></li>
</ul>
</li>
<li><p><a class="reference internal" href="#config-reference" id="id9">CONFIG REFERENCE</a></p></li>
</ul>
</li>
</ul>
</nav>
<p>Datacenter-Data-Delivery Network (D3N) uses high-speed storage such as NVMe flash or DRAM to cache
datasets on the access side.
Such caching allows big data jobs to use the compute and fast storage resources available on each
Rados Gateway node at the edge.</p>
<p>Many datacenters include low-cost, centralized storage repositories, called data lakes,
to store and share terabyte and petabyte-scale datasets.
By necessity most distributed big-data analytic clusters such as Hadoop and Spark must
depend on accessing a centrally located data lake that is relatively far away.
Even with a well-designed datacenter network, cluster-to-data lake bandwidth is typically much less
than the bandwidth of a solid-state storage located at an edge node.</p>
<div class="line-block">
<div class="line">D3N improves the performance of big-data jobs running in analysis clusters by speeding up recurring reads from the data lake.</div>
<div class="line">The Rados Gateways act as cache servers for the back-end object store (OSDs), storing data locally for reuse.</div>
</div>
<section id="architecture">
<h2><a class="toc-backref" href="#id2" role="doc-backlink">Architecture</a><a class="headerlink" href="#architecture" title="Permalink to this heading"></a></h2>
<p>D3N improves the performance of big-data jobs by speeding up repeatedly accessed dataset reads from the data lake.
Cache servers are located in the datacenter on the access side of potential network and storage bottlenecks.
D3Ns two-layer logical cache forms a traditional caching hierarchy <sup>*</sup>
where caches nearer the client have the lowest access latency and overhead,
while caches in higher levels in the hierarchy are slower (requiring multiple hops to access),
The layer 1 cache server nearest to the client handles object requests by breaking them into blocks,
returning any blocks which are cached locally, and forwarding missed requests to the block home location
(as determined by consistent hashing) in the next layer.
Cache misses are forwarded to successive logical caching layers until a miss at the top layer is resolved
by a request to the data lake (Rados)</p>
<p><sup>*</sup> currently only layer 1 cache has been upstreamed.</p>
<p>See <a class="reference external" href="https://massopen.cloud/research-and-development/cloud-research/d3n/">MOC D3N (Datacenter-scale Data Delivery Network)</a> and <a class="reference external" href="https://research.redhat.com/blog/research_project/d3n-multilayer-cache/">Red Hat Research D3N Cache for Data Centers</a>.</p>
</section>
<section id="implementation">
<h2><a class="toc-backref" href="#id3" role="doc-backlink">Implementation</a><a class="headerlink" href="#implementation" title="Permalink to this heading"></a></h2>
<ul class="simple">
<li><p>The D3N cache supports both the <cite>S3</cite> and <cite>Swift</cite> object storage interfaces.</p></li>
<li><p>D3N currently caches only tail objects, because they are immutable (by default it is parts of objects that are larger than 4MB).
(the NGINX <a class="reference external" href="../rgw-cache/">RGW Data cache and CDN</a> supports caching of all object sizes)</p></li>
</ul>
<section id="requirements">
<h3><a class="toc-backref" href="#id4" role="doc-backlink">Requirements</a><a class="headerlink" href="#requirements" title="Permalink to this heading"></a></h3>
<ul class="simple">
<li><p>An SSD (/dev/nvme,/dev/pmem,/dev/shm) or similar block storage device, formatted
(filesystems other than XFS were not tested) and mounted.
It will be used as the cache backing store.
(depending on device performance, multiple RGWs may share a single device but each requires
a discrete directory on the device filesystem)</p></li>
</ul>
</section>
<section id="limitations">
<h3><a class="toc-backref" href="#id5" role="doc-backlink">Limitations</a><a class="headerlink" href="#limitations" title="Permalink to this heading"></a></h3>
<ul class="simple">
<li><p>D3N will not cache objects compressed by <a class="reference external" href="../compression/">Rados Gateway Compression</a> (OSD level compression is supported).</p></li>
<li><p>D3N will not cache objects encrypted by <a class="reference external" href="../encryption/">Rados Gateway Encryption</a>.</p></li>
<li><p>D3N will be disabled if the <code class="docutils literal notranslate"><span class="pre">rgw_max_chunk_size</span></code> config variable value differs from the <code class="docutils literal notranslate"><span class="pre">rgw_obj_stripe_size</span></code> config variable value.</p></li>
</ul>
</section>
</section>
<section id="d3n-environment-setup">
<h2><a class="toc-backref" href="#id6" role="doc-backlink">D3N Environment Setup</a><a class="headerlink" href="#d3n-environment-setup" title="Permalink to this heading"></a></h2>
<section id="running">
<h3><a class="toc-backref" href="#id7" role="doc-backlink">Running</a><a class="headerlink" href="#running" title="Permalink to this heading"></a></h3>
<p>To enable D3N on an existing RGWs the following configuration entries are required
in each Rados Gateways ceph.conf client section, for example for <code class="docutils literal notranslate"><span class="pre">[client.rgw.8000]</span></code>:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">[</span><span class="n">client</span><span class="o">.</span><span class="n">rgw</span><span class="mf">.8000</span><span class="p">]</span>
  <span class="n">rgw_d3n_l1_local_datacache_enabled</span> <span class="o">=</span> <span class="n">true</span>
  <span class="n">rgw_d3n_l1_datacache_persistent_path</span> <span class="o">=</span> <span class="s2">&quot;/mnt/nvme0/rgw_datacache/client.rgw.8000/&quot;</span>
  <span class="n">rgw_d3n_l1_datacache_size</span> <span class="o">=</span> <span class="mi">10737418240</span>
</pre></div>
</div>
<p>The above example assumes that the cache backing-store solid state device
is mounted at <cite>/mnt/nvme0</cite> and has <cite>10 GB</cite> of free space available for the cache.</p>
<p>The persistent path directory has to be created before starting the Gateway.
(<code class="docutils literal notranslate"><span class="pre">mkdir</span> <span class="pre">-p</span> <span class="pre">/mnt/nvme0/rgw_datacache/client.rgw.8000/</span></code>)</p>
<p>If another Gateway is co-located on the same machine, configure it’s persistent path to a discrete directory,
for example in the case of <cite>[client.rgw.8001]</cite> configure
<code class="docutils literal notranslate"><span class="pre">rgw_d3n_l1_datacache_persistent_path</span> <span class="pre">=</span> <span class="pre">&quot;/mnt/nvme0/rgw_datacache/client.rgw.8001/&quot;</span></code>
in the <code class="docutils literal notranslate"><span class="pre">[client.rgw.8001]</span></code> ceph.conf client section.</p>
<p>In a multiple co-located Gateways configuration consider assigning clients with different workloads
to each Gateway without a balancer in order to avoid cached data duplication.</p>
<blockquote>
<div><p>NOTE: each time the Rados Gateway is restarted the content of the cache directory is purged.</p>
</div></blockquote>
</section>
<section id="logs">
<h3><a class="toc-backref" href="#id8" role="doc-backlink">Logs</a><a class="headerlink" href="#logs" title="Permalink to this heading"></a></h3>
<ul class="simple">
<li><p>D3N related log lines in <cite>radosgw.*.log</cite> contain the string <code class="docutils literal notranslate"><span class="pre">d3n</span></code> (case insensitive).</p></li>
<li><p>low level D3N logs can be enabled by the <code class="docutils literal notranslate"><span class="pre">debug_rgw_datacache</span></code> subsystem (up to <code class="docutils literal notranslate"><span class="pre">debug_rgw_datacache=30</span></code>)</p></li>
</ul>
</section>
</section>
<section id="config-reference">
<h2><a class="toc-backref" href="#id9" role="doc-backlink">CONFIG REFERENCE</a><a class="headerlink" href="#config-reference" title="Permalink to this heading"></a></h2>
<p>The following D3N related settings can be added to the Ceph configuration file
(i.e., usually <cite>ceph.conf</cite>) under the <code class="docutils literal notranslate"><span class="pre">[client.rgw.{instance-name}]</span></code> section.</p>
<dl class="std confval">
<dt class="sig sig-object std" id="confval-rgw_d3n_l1_local_datacache_enabled">
<span class="sig-name descname"><span class="pre">rgw_d3n_l1_local_datacache_enabled</span></span><a class="headerlink" href="#confval-rgw_d3n_l1_local_datacache_enabled" title="Permalink to this definition"></a></dt>
<dd><blockquote>
<div><p>Enable datacenter-scale dataset delivery local cache</p>
<dl class="field-list simple">
<dt class="field-odd">type<span class="colon">:</span></dt>
<dd class="field-odd"><p><code class="docutils literal notranslate"><span class="pre">bool</span></code></p>
</dd>
<dt class="field-even">default<span class="colon">:</span></dt>
<dd class="field-even"><p><code class="docutils literal notranslate"><span class="pre">false</span></code></p>
</dd>
</dl>
</div></blockquote>
</dd></dl>

<dl class="std confval">
<dt class="sig sig-object std" id="confval-rgw_d3n_l1_datacache_persistent_path">
<span class="sig-name descname"><span class="pre">rgw_d3n_l1_datacache_persistent_path</span></span><a class="headerlink" href="#confval-rgw_d3n_l1_datacache_persistent_path" title="Permalink to this definition"></a></dt>
<dd><blockquote>
<div><p>path for the directory for storing the local cache objects data</p>
<dl class="field-list simple">
<dt class="field-odd">type<span class="colon">:</span></dt>
<dd class="field-odd"><p><code class="docutils literal notranslate"><span class="pre">str</span></code></p>
</dd>
<dt class="field-even">default<span class="colon">:</span></dt>
<dd class="field-even"><p><code class="docutils literal notranslate"><span class="pre">/tmp/rgw_datacache/</span></code></p>
</dd>
</dl>
</div></blockquote>
</dd></dl>

<dl class="std confval">
<dt class="sig sig-object std" id="confval-rgw_d3n_l1_datacache_size">
<span class="sig-name descname"><span class="pre">rgw_d3n_l1_datacache_size</span></span><a class="headerlink" href="#confval-rgw_d3n_l1_datacache_size" title="Permalink to this definition"></a></dt>
<dd><blockquote>
<div><p>datacache maximum size on disk in bytes</p>
<dl class="field-list simple">
<dt class="field-odd">type<span class="colon">:</span></dt>
<dd class="field-odd"><p><code class="docutils literal notranslate"><span class="pre">size</span></code></p>
</dd>
<dt class="field-even">default<span class="colon">:</span></dt>
<dd class="field-even"><p><code class="docutils literal notranslate"><span class="pre">1Gi</span></code></p>
</dd>
</dl>
</div></blockquote>
</dd></dl>

<dl class="std confval">
<dt class="sig sig-object std" id="confval-rgw_d3n_l1_eviction_policy">
<span class="sig-name descname"><span class="pre">rgw_d3n_l1_eviction_policy</span></span><a class="headerlink" href="#confval-rgw_d3n_l1_eviction_policy" title="Permalink to this definition"></a></dt>
<dd><blockquote>
<div><p>select the d3n cache eviction policy</p>
<dl class="field-list simple">
<dt class="field-odd">type<span class="colon">:</span></dt>
<dd class="field-odd"><p><code class="docutils literal notranslate"><span class="pre">str</span></code></p>
</dd>
<dt class="field-even">default<span class="colon">:</span></dt>
<dd class="field-even"><p><code class="docutils literal notranslate"><span class="pre">lru</span></code></p>
</dd>
<dt class="field-odd">valid choices<span class="colon">:</span></dt>
<dd class="field-odd"><ul class="simple">
<li><p><code class="docutils literal notranslate"><span class="pre">lru</span></code></p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">random</span></code></p></li>
</ul>
</dd>
</dl>
</div></blockquote>
</dd></dl>

</section>
</section>



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